The present disclosure generally relates to electronic displays, and specifically relates to directed electronic displays in head-mounted displays (HMDs).
Conventional display technology is designed to have a large viewing angle. This is usually done by designing the display to act as a Lambertian source. A Lambertian source is defined as a source whose emission distribution is such that the apparent brightness (or luminance) is independent of angle—in other words, the off-axis luminance is the same as the on-axis. A Lambertian source (or a source with a similar emission distribution) helps minimize any variation in emitted optical light as a function of viewing angle. This is quite useful for traditional consumer electronics that are designed to be used in environments where viewers can be located at different positions relative to the display and the position of the viewer relative to the display does not cause large changes in apparent brightness.
But in the space of HMDs, a Lambertian emission distribution is largely a hindrance, as a viewing user is located at a specific position relative to the display. As a Lambertian emission pattern is designed to emit light to maximize viewing angle, a lot of the off-axis light is wasted (as it is not being directed to eyes of the user wearing the HMD, notionally located in a region defined as the “eyebox”), and therefore power used to generate the light is also wasted. Moreover, the wasted light introduces a potential for stray light, as the wasted light can scatter within the HMD and reduce the apparent contrast of media being presented by the HMD.
Moreover, HMDs may magnify an internal display to, e.g., increase a field of view for the viewing user. However, conventional displays are traditionally not meant for magnified viewing (above normal unaided vision perception limits). Accordingly, the increased magnification (versus unaided viewing) of a conventional display in a HMD can lead to the viewing user seeing the “dead-space” between the emission points of the display, which causes a “pixelated” or “screen-door” effect under use.